Welcome. 350' is great so soon in the sport but 400' in the first month is phenomenal. So you are well off in the start indeed.

You have too long steps for that speed and not enough left leg forward push so you lean too far back when the disc leaves. You should stop the right leg in the final step in place for a while when the heel hits the ground and the ball of the foot rises up. Because that would allow you to twist the hips to the right of neutral. The shoudlers should twist even more. A stopped in place right leg helps in the passive twist of the hips brought by momentum of the steps and stopping of the right leg but you can add even more power by twisting with core muscles. Your right leg spins upon landing of the final step of the x steps and that turns your body toward the target too early reducing how far the elbow travels ahead of the body. That is reducing the elbow bending angle which in turn reduces power generation a lot. The straightening of the elbow is way more explosive with a tight bend. The elbow bends tighter the more it goes forward of the body bringing the disc to the right side or closer to the target. The problem is that if you push the elbow as far forward as possible it can lock up and the moemtum twists the elbow open against the natural motion range stretching tendons and muscles more than they should. It hurts like hell and can maim you for life so for the sake of safety never extend the upper arm to elbow as far forward as you can. You should remain an inch or more short of that.

When you get from the reach back to the ripping out of the disc the steps have enough moemntum to turn the body to face the left side of the tee and getting the disc to the left side without moving the arm at all. Your power dictates whether you should start to move the arm then, earlier or later. Later means little power and since you were at 400' i don't think that is the case for you. It is not mandatory to push with the left leg just yet but the timing here too is determined by how fast you were moving and how much left leg push power you have. The body should not turn toward the target that wuickly for the time it takes to move the disc from the left side to the right or a little beyond. Then the left leg push should turn you toward the target.

You could have the left arm closer to the body during the throw.

Flat shots need running on the center line of the tee and planting each step on the center line. Anhyzer needs running from rear right to front left with the plant step hitting the ground to the left of the line you're running on. Hyzer is the mirror of that.